by Pietro Vischia

Today I had a very full day. It started with a working meeting (an informal meeting of the people working on some analysis) for planning how to improve the EWkino analysis (one of the searches for new physics we are involved in) with the new data CMS is taking. I then went on to the UniOvi CMS group weekly meeting, in which we all meet in the large common office (plus some people connected in videoconference from CERN) to discuss the various topics the group is involved it. Let me delay to another occasion my rant on the amount of CMS meetings we are subjected to!

Then, after some quick and frenetic coding work, I had lunch, and a meeting with the three new students that are going to spend the next months working for their BSc with us.

Finally, I got out of the office at 5 PM, which is very unusually early: how comes it had been a full day?

Well, today was the European Researchers Night, so I got out of the office bringing the group’s diffusion cloud chamber, that is a very simple particle detector designed to visualize the passage of ionizing particles through a stream of condensation bubbles in a small chamber filled up with alcohol in vapor form. The bottom of the chamber is cooled to roughly -35° C, and the temperature gradient with respect to the top of the chamber (that is at ambient temperature) creates a supersaturated vapor environment in the region of the chamber near the bottom.

When an ionizing particle (an energetic charged particle, usually coming from the interaction of a primary cosmic ray — coming from the stars — with the higher Earth atmosphere) passes through the supersaturated vapor, it ionizes it, and the ions work as condensation centers for the gas: a quantity of small liquid bubbles form across the trajectory of the ionizing particle, enabling us to see its trajectory with our eye.

If you want to build your own simple cloud chamber, don’t hesitate: do it, it’s actually pretty simple!!!

From the right: Pablo, Francisco, Miguel, Javier, me

I was in charge of the stand of Cielos Despejados (Clear Skies), an outreach association I am part of, devoted to the observation of the sky with the association’s telescope and to scientific dissemination about the cosmos. Being also a representative of the UniOviHEP group, I brought also the cloud chamber, and mounted it in the stand in the cloister of the nice historical building of the Universidad de Oviedo.

Our stand was the one in the red circle.

The photo to the right was taken from the Tower of the building, where we had mounted the association’s telescope, with the plan of attaching a camera to it, and transmitting the signal to the stand below, that was equipped with a very large TV screen.


Unfortunately, we had huge network issues, and rain started pouring down, so we had to climb down and bring the telescope to the stand, which implied having a very limited field of view: we actually spent the night having visitors look — through the telescope, to the watch mounted on top of the tower. In the photo, you can see the mechanism driving the tower watch!

Photo credit:

We decided to use the TV screen to project with a computer the simulation of the sky provided by the very nice software Stellarium, which is a very powerful and flexible planetarium software with many cool functionalities. Plus, it’s free software, released under the GNU license (in the photo, Richard Stallman being happy)!!!



IMG_20170929_210449At this point, Miguel (a first year student from the BSc course in physics) took the microphone and started explaining to the visitors (lots of children!!!) a lot of things about the observable sky using the Stellarium simulation (as the real sky was covered by a thick layer of clouds, plus the rain). I swear: he talked for almost three hours without stopping for a second! He was amazing!!!

After an hour or so, when the light started to fade, I finally mounted the cloud chamber: I had to wait, because even if the interior of the chamber is illuminated, it is a bit difficult to observe the bubbly traces if there is too much light in the surroundings. In the photo, you can see the bubble chamber, and the TV screen with Stellarium.

The chamber is connected, through a pump and some tubes, to a bucket filled with water and ice: this has the double purpose of giving the chamber’s internal Peltier cell a low starting temperature, and dissipating the heat produced by the Peltier cell (a Peltier cell is a thermoelectric devise that is used to cool things down, at the price of heating up the rest of the device). The Peltier cell is the responsible for lowering the temperature in the bottom of the bucket to -35° C.

While I was still mounting the chamber, a lot of children stormed by, asking what I was doing, and what was that lighted up device, so I briefly explained them the purpose of the device and the way it worked, but had to tell them to stop by at a later time for actually seeing the chamber in action: this is because the device needs to heat up before reaching the working regime (it takes about half an hour).

At some point, I was able to shoot a brief video of the chamber in action, and you can watch it below! Most of the tracks are from muons (in the volume of the chamber, we expect about 3-4 muons per second), then there are a few electrons (often seen as V-shaped tracks: what happens there is that a photon enters the chamber, invisible, and then converts into two electrons, thus generating the V-shaped bubble tracks.

In the short time span of the video, I don’t remember having seen any alpha particles passing by: these are helium nuclei, hence heavier, larger, slower, with more charge than the electrons or muons; this implies that they ionize more the alcohol gas, and so their passage would be announced by a very thick (up to ~1 cm) bubbles stream).


Cool, isn’t it?

People were very responsive to the instrument and the explanation of its inner workings! The cool thing of the European Researchers Night in Oviedo is that lots of families with small children tag along!

Some children even spent the night going in groups from stand to stand to work on assignments from their school teachers!!! I think this is super cool!!! Two or three groups actually even requested to record my description of the experiment, for their later processing: of course I accepted, but when people are recording you then you have always this lingering tension…

Then there was this kid that asked for all the details, and then asked my informed opinion on whether it would be easy to build a replica of the cloud chamber in the open, in the middle of a grass field, and did not stop until I explained him thoroughly why it would be unattainable.

Then, a girl asked me how you decide your research field «among all the cool things you can do in physics and mathematics», which is very awesome! I gave her a few examples of some friends I have and mine, and finally encouraged her to pursue whatever field provides the most fun to her!

And finally, the super-cool event of the night: a girl, seemingly about 7 or 8 years old, listened to the cloud chamber explanation, asked a few questions to understand better a few details (children are smart! They always ask the best questions!). Then she remained silent some 10 seconds, intently watching the cloud chamber, with this serious and critic facial expression.

Finally, she looked up, nodded solemnly,  and very-very-seriously said: «Es chulo!» (It’s cool!), and marched away, leaving me utterly flabbergasted.

Because this, ladies and gentlemen, is ultimately why we do science, and why we crawl out of our offices to show the science to you.

Because it’s fun, and chulo.